Quantum information can't break the cosmic speed limit, according to researchers* from the National Institute of Standards and Technology (NIST) and the University of Maryland's Joint Quantum Institute. The scientists have shown how attempts to "push" part of a light beam past the speed of light results in the loss of the quantum data the light carries. The results could clarify how noise might limit the transfer of information in quantum computers.

The speed of light in vacuum is often thought to be the ultimate speed limit, something Einstein showed to be an unbreakable law. But two years ago,** members of the research team found a sort of "loophole" in the law when they devised a new way to push part of the leading edge of a pulse of light a few nanoseconds faster than it would travel normally. While the 'pulse front' (the initial part of the pulse) still traveled at the usual constant speed, the rising edge and the pulse peak could be nudged forward a bit. Since waves carry information, the team decided to explore what their previous results might mean for quantum information.

"How does the beam's quantum information behave if you try to speed up the leading edge?" says NIST's Ryan Glasser. "We knew if you could speed the information up successfully, it would give rise to all kinds of causality problems, as you see in science fiction movies about people traveling back in time. So while no one expects it to be possible, just what prevents it from happening? That's what we wanted to know."

The team set up a new experiment that "entangled" the photons in two different light beams, which means that quantum information in one beam—such as amplitude—is strongly correlated to information in the other. Ordinarily, measuring these parameters in one beam can reveal those in the second. But when the team nudged the waves in one beam forward and took their measurements, they found the correspondence with the second beam started to taper off, and the more they pushed, the more degraded with noise the signal became.

"We sped up the peak of the correlation between the two beams," Glasser says, "but we couldn't push the quantum information any faster than the speed of light in vacuum."

While further work is needed to determine what is fundamentally enforcing this information speed limit, the current findings could be useful for understanding information transfer within quantum systems such as those that will be needed within quantum computers. "We speculate that quantum noise and distortion set that limit," Glasser says.

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